The testing of flowable solid materials such as soil, sand and liquids for their mechanical properties such as shear strength and viscosity has long been an important aspect in the design of, for example, building foundations, earthworks, underground pipe installations, new liquid products such as paints, cosmetics and foodstuffs. This is usually done by deforming samples of the materials and measuring the corresponding bulk relationships connecting nominal stress, strain, and their rates of change. Bulk properties such as the material strength and viscosity are then obtained from these relationships. Several devices with various geometries already exist and are in routine use. For example, the tri-axial apparatus, the Couette rheometer and the plane-Couette rheometer are described below.
FIG. 1 shows a known device for determining material properties, being a tri-axial apparatus 1 in which the material 2 to be tested is supported on a base 3 and is sealed within a rubber membrane 7 that is surrounded by water 4 in a confining cylinder 5. A load 6 is applied to the material from above. A disadvantage of this device is that the material 2, for example soil, bulges within the rubber membrane 3 as it is compressed. Furthermore, the soil tends to fail along a localised failure zone, or plane, within the bulk material causing the rubber membrane 3 to further shift and deform. As a result, the movement of the rubber membrane 3 interacts with the soil deformation. As a result, the material behaviour along the localised failure cannot be isolated and measured and bulk measurements of the material sample cannot provide reliable data about the properties determining the emergence of the localised failure. A disadvantage of this device is thus that it is limited to only measuring material properties undergoing low levels of deformation that do not cause material failure.
FIG. 2 shows another known device for material properties, being a cylindrical Couette shear device 10. The Couette shear device 10 comprises an inner cylinder 11, a concentric outer cylinder 12 and an annular testing space 13 formed between the two cylinders. The material to be tested is placed in the annular space 13 and the outer cylinder 12 is rotated relative to the inner cylinder 11. The rotation creates a continuous shear strain within the material. However, due to the cylindrical geometry, the stresses produced within the material diminish radially from the inner cylinder 11 towards the outer cylinder 12. As such, the stresses and the strain rate are intrinsically heterogeneous. The device thus has the same disadvantage as the tri-axial apparatus, in that the bulk measurement of strain cannot accurately reflect the true strain gradients within the bulk material due to the non-homogeneity of the material deformation.
A further known device is a plane Couette device (not shown), which comprises a pair of rotating cylinders and a flexible belt arranged around the cylinders. The belt and cylinders are placed inside a tank and the entire apparatus is filled with water. The belt is driven by one of the cylinders to create a shear flow inside the belt. This device produces a continuous shear flow inside the belt, but it has the disadvantage that the fluid sample is allowed to flow both inside and outside the belt. Any measurement of the shear strength and viscosity of the fluid is therefore not representative for a fixed material sample. This ‘open’ system also makes it almost impossible to control the confining stresses applied to the fluid. This is a major issue for most of the complex materials mentioned above, since the confining stresses play a crucial role in determining their mechanical behaviour and thus interpreting their properties.